This invention relates generally to color television systems that carry the picture information by means of a luminance signal and a number of color difference signals (usually two), and more particularly to a technique for compensating for the violation of the constant luminance principle in color television systems.
During the past several years there has been considerable emphasis on improving or enhancing the performance of the NTSC color television system. Significant improvements in picture quality can be obtained in all stages of the NTSC signal generation, distribution and display, with most progress, up until now, having been in the area of signal generation and display. However, composite signal distribution has always been considered the bottleneck of the NTSC system and several proposed solutions avoid the bottleneck by departing from the NTSC composite color signal fundamental to the system and have gone to the distribution of components, be it MAC, SMAC, TMC, etc.
Even though the NTSC color encoded format suffers from some inherent limitations which cause deterioration of picture quality, the system will be with us for many more years, therefore making it worthwhile to compensate for or reduce the defects that result from such inherent limitations.
It is well known that the NTSC color encoded signal consists of wideband luminance and two band-limited color difference signals. Therefore, it is inherent with this format that high frequency details will only be carried as monochromatic information, that is, they are considered black and white signals. This is done by design and in principle is generally acceptable because the perception of the eye to color drops off quite rapidly at high spatial frequencies. However, there is a problem due to the fact that the luminance level for those high frequency details is correct only when the source is monochromatic, and it is always less than what it should be to different degrees, when the high frequency details are derived from or are supposed to represent a particular color. This difficulty arises from the fact that prior to luminance matrixing, each color is gamma correct, usually with a gamma of 1/2.2, in order for each color to be properly displayed on a cathode ray tube (CRT) display, which has a nominal gamma of 2.2. If the system is normalized to operate with a signal level from 0 to 1, gamma correction means that the corrected signal is equal to the input signal raised to the gamma power; i.e., the output is equal to the input to the exponent gamma. Thus, to compensate for the nominal CRT gamma of 2.2, a luminance level of 0.11 actually requires a signal level of 0.37; a luminance level of 0.30 requires a signal level of 0.58; a luminance level of 0.59 requires a signal level of 0.79; and a luminance level of 1.0 requires a signal level of 1.0.
A typical matrix used to generate the luminance signal from the blue, red, and green color signals derived from a color television camera, for example, is given by the equation Y=0.11B+0.30R+0.59G, where Y is the luminance signal, B is the blue signal, R is the red signal, and G is the green signal. As previously mentioned, most of today's television systems use gamma corrected B, R and G signals to generate luminance. Considering, then, what happens if, for example, the source signal should contain only blue information at 100% level, that gamma corrected blue signal would have a normalized level of 1.0 and, accordingly, would contribute a value of 0.11.times.1.0 to the luminance signal. However, for a luminance value of 0.11, based on the gamma characteristic of a CRT, a signal level of 0.37 is actually required; thus, the obtained luminance level is in error by more than 10 dB. A similar condition will result if the source signal contains only a red signal, say at 100% level; it would contribute 0.30.times.1.0 to the luminance, whereas a signal level of 0.58 is required for proper luminance display. Thus, in this case, the luminance is in error by about 5.7 db. Similarly, if the source signal contained only green, at 100% level, the resultant luminance would be 0.59.times.1.0, whereas for proper display it should have been 0.79; thus, there would be a 2.5 db error. The phenomenon illustrated by these examples is known as the violation of the constant luminance principle. A similar analysis can be carried out for any combination of non-equal red, blue and green signals, and it can be shown that the only time the constant luminance principle is not violated is when the source signal has equal blue, red and green signal levels which, of course, indicates black and white picture information.
It is well known that these errors can be avoided by generating the luminance signal from linear color signals, i.e., color signals that are not gamma corrected. Unfortunately, if that were done the luminance signal would be incompatible with just about any television system or television receiver in current use, the great majority of which are designed to accept gamma corrected signals. Although at the lower frequencies, where color difference signals as well as the luminance signal are available, the receiver matrices that combine the luminance and the color difference signals to regenerate individual color signals automatically cancel out the violation of the constant luminance principle, at the higher frequencies, where only luminance information is available, the receiver has no means of correcting for constant luminance violations. Therefore, for proper picture presentation it is necessary to pre-correct, at the signal source or at the encoder, for the violation of the constant luminance principle. Previously proposed techniques for doing this of which applicant is aware, were reviewed in an article published in the Journal of the SMPTE, Volume 94, No. 7, July 1985, consist essentially in generating a correction signal based on complex calculations to determine the constant luminance violation error. Another known approach uses gamma corrected colors to generate the low frequency luminance signal and uses linear color signals to generate high frequency luminance, that is, luminance having frequencies beyond the passband of the color difference signals. Although these techniques have been shown to have merit, they are very complex and therefore costly to implement, they are often inaccurate and unstable, and to applicant's knowledge have not yet been used in any commercially available system.
A primary object of the present invention is to compensate, to a first approximation, for the above-mentioned violations of the constant luminance principle. Another object is to provide a very predictable and stable, and relatively simple, circuit for achieving compensation for violations of the constant luminance principle. An additional object is to provide a system for overcompensating or enhancing the luminance signal as a function of each individual color.